DIPSY: A new Disc Instability Population SYnthesis

Context . Disc instability (DI) might provide an explanation for the formation of some observed exoplanets. At the same time, our understanding of this top-down formation mechanism remains limited. Existing studies have made strong simplifications, and the predicted population is poorly known. Aims . We aim at overcoming several limitations and produce a more advanced synthetic population of companions formed via DI that can be used for quantitative statistical comparisons with observations, and to make predictions for future surveys. Methods . We applied the global end-to-end model described in Paper I of this series to perform a population synthesis of companions formed via DI. By using initial conditions compatible with both observations and hydrodynamical simulations, and by studying a large range of primary masses (0.05 M $_{⊙}$ to 5 M $_{⊙}$ ), we can provide quantitative predictions of the outcome of DI. Results . In the baseline population, we find that ~10% of the discs fragment, and about half of these end up with a surviving companion after 100 Myr. Based on their mass, 75% of the companions are in the brown dwarf regime, 15% are low-mass stars, and 10% planets. At distances larger than ~100 AU, DI produces planetary-mass companions on a low percent level. Inside of 100 AU, however, planetary-mass companions are very rare (low per mill level). The average companion mass is ~30 M $_{♃}$ scaling weakly with stellar mass. Very few companions of all masses reside inside of 10 AU; outside this distance, the distribution is approximately flat in log-space. Eccentricities and inclinations are significant, with averages of 0.4 and 40 $^{°}$ . In systems with surviving companions, there is either one (80%) or two (20%) companions. The fraction of surviving synthetic brown dwarfs is consistent with observations, while that of planets is lower than observed. Most of the initial fragments do not survive on a Myr timescale; they either collide with other fragments or are ejected, resulting in a population of free-floating objects (about 1–2 per star). We also quantify several variant populations to critically assess some of our assumptions used in the baseline population. Conclusions . DI appears to be a key mechanism in the formation of distant companions with masses ranging from low-mass stars down to the planetary regime, contributing, however, only marginally to planetary mass objects inside of 100 AU. Our results are sensitive to a number of physical processes, which are not completely understood. Two of them, gas accretion and clump-clump collisions, are particularly important and need to be investigated further. Magnetic fields and heavy-element accretion have not been considered in our study, although they are also expected to affect the inferred population. We suggest acknowledging the importance of the gravito-turbulent phase, which most protoplanetary discs experience. Exploring hybrid DI – core accretion scenarios, and quantitative comparisons of theory and observations will improve our understanding of star and planet formation.